There are a number of browsers allowing you to switch between rendering engines, with Lunascape being one of them. We've covered this browser before quite extensively, and today, the Lunascape team released the first beta of Lunascape 6 ORION, the bext version of this triple-engine web browser.

It follows the idiocy of - Hey! The more the better!
Users will go like It must be at least 3x better!.

If you want more browsers, it's a much better choice having separate clean installs. A actualized browser is a major software achievement. Now I don't see the point of handling 3 different engines just for the sake of it.